230 N. K. TOLBERT AND L. P. ZILL 



Discussion 



Gibbs : I want to coranient on the glycolic acid excretion, .since I remember that 

 at the botanical meetings last summer Dr. Ranson had some very interesting data 

 on this point. He showed that with normal CO2 concentrations and Bnjophyllum 

 one obtains a typical radiochromatogram as shown by many workers. However, as 

 one increases COs concentration up to 30% and 40% the compounds which one 

 normally finds labeled, that is, the free sugars, sugar phosphates, etc., disappear 

 and the only compound which contains tracer is malic acid. As a consequence 

 one might say the carboxylating enzyme is apparently more sensitive to CO2 

 concentration than the malic enzyme. One wonders if the ribulose diphosphate 

 that accumulates is partially converted to a diose which cannot carboxylate 

 since the carboxylation enzyme is inhibited so that it spills out of the cell as gly- 

 colic acid. 



Tolbert: Ranson's work was at 30% to 40% CO2 partial pressures and he did 

 not report glycolic formation under those conditions. The bicarbonate-glycolate 

 shift occurs at low CO2 partial pressure, in the range of 1% CO: to 0.3% or lower. 

 Even at 2, 3, or 4% CO2 in air, variations in the path of the carbon have not been 

 reported for Chlorella. Thus glycolic formation and excretion would not appear to 

 arise from abnormal breakdown of ribulose diphosphate, but rather this is a nor- 

 mal pathway of metabolism from the pentose phosphates. 



Gibbs: If one incubates pentose phosphate with (I hate to mention the word) 

 cell-free preparations under anaerobic conditions, the pentose phosphate is con- 

 verted to hexose phosphate. However, we found this summer if one does this under 

 aerobic conditions the diose can be split off and converted to glycolic acid. We 

 are attempting to purify this enzyme which takes a C2 piece away from pentose 

 phosphate and causes oxidation to form glycolic acid. Thus there is good evi- 

 dence that glycolic acid can be formed directly from the so-called "active glycol- 

 aldehyde" piece of pentose phosphate. 



Aronoff: It is interesting to note the excretion of glycolic acid during photo- 

 synthesis. However measurements on CO2 exchange by roots, which presumably 

 would be in a high atmosphere of CO2, showed the presence of considerable gly- 

 colic acid in the root. 



Tolbert : With roots the net exchange is that of bicarbonate or CO2 arising from 

 respiration and passing out of the root. According to our hypothesis of bicarbon- 

 ate-glycolate shift, there should thus result an accumulation of glycolate inside 

 the root cells. 



M. B. Allen: Excretion of organic acids by Chlamydomonas might be of interest 

 here, although this is not a transient phenomenon. In contrast to Chlorella, 

 which excretes glycolic acid and then reabsorbs it, growing cultures of six species 

 of Chlamydomonas have been found to accumulate soluble organic material in 

 the culture medium (Allen, M. B., Arch. Microbiol., in press). Formation of 

 soluble products paralleled the growth of the alga and was favored by high light 

 intensity and nitrate as nitrate source. Ten to forty per cent of the organic ma- 

 terial synthesized by the algae appeared in the medium in soluble form. 



Examination of the culture filtrates showed that the soluble organic material 

 consisted partly of polysaccharide and partly of organic acids. The acids included 

 glycoHc, oxalic, and a keto acid which may be pyruvic. 



